Electrical testing and indicating system



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Sept. *27,- 1949. E. c. PYATT ErAL 2,482'932 ELECTRICAL TESTING AND INDIGATING SYSTEM Filed July 29. 1947 5 'Sheen-sheet s Patentecl Sept. 27) 1949 ELEcraIoAL TESTING AND mDIoA'rING Vsys-ren Edward Charles Pya'tt and Kenneth iCedc Arthur King, London, England, Telephone Manufacturing Company London, England, a British company assignors to i Limited, V

Application July 29, 1947, Serial No. 764,528 I In Great Britain July 24, 1946 Section 1, Public Law 690, August 8, 1946 Patent expires July 24, 196,6,

sclaims. (cl. 178-69) I j This invention relates to electrical testing and indicating systems. The invention is more particularly concerned With systems for testing or indicating the operation of telegraph apparatus.

-In certaintypes of telegraph systems, use is made of signals comprising mark and space elements of Which the duration is a multiple of a fixed unit. In practice, however, the received signals are distorted and depart from the exact spacing, and for various purposes, such as the marginal testing of teleghaph apparatus, it is desirable to know the degree of distortion in an actual or test signal. In systems of this type, it is customary to indicate distortion by adding the greatest error of time by Which any signal transition, that is a change from mark to spaceV or space to mark, precedes the correct transition point, to the greatesterror by Which any signal transition follows the correct transition point and expressing the result as a percentage of the fixed unit.

One method of measuring signal distortion and Which aifords direct readings on meters is one in Which the final indicating circuit is dependent on measuring the charge produced on a condenser by the signal, this charge being proportional to distortion or peak distortion. This method has certain disadvantage; it is dependent upon the leakage resistance of the condenser and the shunt resistance of the measuring circuit and there is a definite limit of time after the start of the measurement, after which .observations of sufiicient accuracy cannot be made, so tiat the possible length of any test signal is limi ed.

The invention has for its object to provide an improved apparatus for indicating percentage distortion of signals of this kind and especially one in Which a readily readable indication may be obtained, for example by means of a meter, and without the distadvantages of the earlier methods referred to.

In a system according to the present invention, a series of timing impulses are produced, one of which may be made coincident with a sig-.y

'nal transition. If subsequently a signal transition occurs and does not coincide With a timing pulse, that is, if the signal contains a distorted element, means are provided to lproduce measuring pulses spaced by a duration equal to the error. If subsequently, there is a greater errory in the same sence, or an error in the opposite sense, the spacing of the measuring pulses is accordingly increased so that it is equal to the sum of the maximumerrors, The measuring pulses are caused to operate a ftrig'ger .circuit so as to producesquare waves of flduration propor- 25'devices is Varrangec'l:toI be capable of beingretional to the 'spacng 0fthe,measuring pulses, and means' areprovided responsive' to the duration of the square KWaves'to..pro vide the desired indication. i V In a preferred arrangement according to the invention, use is nadefi of .a standard ltiming device such as a stabilisednoscillator whose, period isa small aliquot part of the fiX-ed time unit of the signal, i. e., the oscillator'*A frequency is a high multiple of :the telegraph frequency. The oscillationsof the oscillatorjare applied to pulsecounting devices which each produce a single output'pulse after receiving the 'number of pulses Which occur'in the fixed time unit. i

Two ,such pulse-counting devices are employed receivinglin parallel. inputl pulses fromthe oscil- 'lator and the counting devices Will, therefore,

each emit aitrainvfofflpulses vgspaced by the fixed timel unit of;the signal. LThe counting devices farranged [to periodically start in response to pulses received from started.Onliafteireceivieg pulse'and the 'otherbefore receiving pulses, where n isV thevnumber ofv oscillations of the stabilised oscillator Which occur in the fixed timel unit. 1 .i I.

VMeans are providedi-Whereby both counters are started by the firstflsignal transition; this transitionis used for. a time reference point. After this instant, one counter is only restarted by a signal transition Which occurs in advance of its Vcorrect positionfinl time and subsequently by a signal transition Which is more in advance .of the-correct position than anyprevious Vtransition.

-.Similarly, :thentherlcounter is only restarted by a vsignal transition Which occurs. later than the correctposition and..subsequently by a signal transition more delayedy than any previous transition.

If either counter is not restarted by a signal ,tia1ly' occurring at the same instant Will, if the signal-is distorted, have a relative displacement in fleeeualisihe. reals-disiatmnaesaoae The separate pulses of each pair are arranged to be of opposite polarity andv are used to operate a trigger cir'cuit ofth'e type which gives an output dependent upon the polarity of the previous input pulse such as is described in our copending application Serial No. 739.876, filed April 7, 1947. a direct-current component proportional to distortion which may be indicated onw a-meter hy conventional circuits. Switching meansare provided so that observationsmay-be made of 'transitions from mark to space or from space to mark as desired.

A more complete understanding of' the invention may be had by reference to the following description and the accompanying drawings .in which Figures la to lg show waveforms at certain points of the circuit, Figure 2 is ablock schematic diagram of a complete arrangernent', a part of which is shown in more detail in Figures 3a, 3b and '3c. i

Figure la shows a typical 'undistorted' telegraph signal with all' transitions occurring at The resultant rectan-'gular-waveform lha'sf time intervals which are a multiple of the telegraph unit, while Figure Ib shows the same signal as it might appear after transmission over a line or through an apparatus givingl rise to distortion. In Figure lb the various transitions have been delayed for unequalV periods of time, so that the mark and space elements no longer have the correct relationship.

In order to measure the time errorof the occurrence of each transition, a series of time ref-V erence points are set up with a recurrence 'frequency equal to the telegraph frequency. If the first of the reference points occurs at' the instant when the first transition of the signal under test occurs, then each succeeding transition should coincide with a reference point.

Transitionswhich do not coincide with a reference point thus indicate distortion and means are, therefore, provided to measure the maximum time difference (te) betweenv the arrival of a transition and its associated reference point when the transition occurs before the reference point, and other means are provided to measure the time difference (tbi between the arrival of a transition and its associated reference point when the transition occurs after the reference point. The sum (tal-ts) expressed as a percentage of the telegraph time limit is then the maximum distortion that has occurred up to the time of the measurement.

It should be pointed out that the degree of distortion which occurs in practice, and the degree of distortion which would normally be produced by a test equipment simulating practical conditions, is not so great as to cause confustion between a transition and its associated reference point,. and in general the associated reference point will be that nearest in time to the transition.

In order to measure the sum of the time intervals between. the arrival of e. transition and the associated reference pulse, two pulse counting circuits A and B are arranged in parallel so as to count cycles from an oscillatorv Operating at a frequency of n times the telegraph frequency. A convenient value for n is 100'; within limits, the higher the value of n the greater is Vthe accuracy of the measurement. These counters are arranged'to deliver an output pulse after receiving n,- pulses and the output pulse rate will, therefore, equal the telegraph frequency.

Each counter has a control circut which can '4 restart the counting process from zero. Means are provided so that counter A can only be restarted before it has counted pulses and counter B can only be restarted after it has counted pulses.. The signal which restarts the counters is derived from the incoming telegraph signal by circuits which produce a sharp voltage pulse at each tra'n'sition. The output from counter A is a series of pulses of polarity opposite to the pulses from counter B.

The pulses are mixed and applied to a trigger cir'cuit of the type described in the previously mentioned copending application which delivers a steady output of which the amplitude depends upon the polarity of the last-pulse received.

The output of this mixer circuit, which is applied to the input of the trigger circuit, win consist of pairs Vof pulses of alternate polarity and spaced by the maximum error and the output of the trigger circuit, therefore, consists of a series of rectangular pulses of constant amplitude and with width equal to the time interval between the most delayed and the least delayed transitions. The width of these pulses is indica'ted by cohnecting the output of the trigger Circuit to a meter indicating the average direct current component. This meter can, therefore, be calibrated directly in percentage distortion.

The waveforms of the voltages at various parts of the apparatus are indicated in Figures la to lg; of these figures, Figure la shows the undistorted signal; Figure lb shows the same signal as it might appear after being distorted; Figure lc shows the output from counter A which can only be restarted before it has counted pulses, and Figure ld Vshows the output from counter B which can be restarted only after it has counted pulses; Figure le shows the waveform obtained from the output of the mixer to which the waveforms of Figures lc and ld are applied and Figure 1 f shows the output of the final trigger Circuit to which voltages as in Figure le are applied; Figure lg shows the voltage indicated by the meter.

The undistorted signal as shown in Figure la has a series of transition points ai, az, ax, ai, and the time intervals aiaz, azaa, osar will all be an integral'number of signal time units. These transition points after distortion by a transmission line or apparatus occur as transition points bi, bz'; ba, br. The intervals bibz, bzbs, bsbi may no longer be an integral number of time units.

The transition which occurs at br is caused to produce a Sharp pulse which is used to restart ansaosa oscillations of the oscillator. Similarly, counter B is'started every n oscillations and can be restarted by a pulse derived from a transition if it occurs after the counter has counted oscillations.

' In the example shown, the second transition bz arrives exactly one time unit after b1, so that the resultant output from counter A and counter B occurs after n oscillations of the master oscillator oscillations. Transition ba, therefore, has no re- :starting effect on counter B but restarts counter A at point cs. It can be seen that the time difference between d and cs is now equal to the distortion of transition ba with respect to the first transition at b1.

It will be seen that pulses cz and dz, cs and da,

Vand' 04 and di occur at the same instants ofV time,

and as they are of opposite polarity the effect of combining them' in the mixer circuit is to produce zero output in the mixer output circuit, as indicated in Figure ie. On the other hand, pulses aa and ds are not coincident, and there is produced in the mixing circuit output a pair of pulses ei, ez, which cause the trigger circuit to produce an output pulse fi. A

Intervals cs, cv and da'ds are each equal to unit time, so that pulses cv and ds' give rise to apair of pulses es, er which are spaced by the same amount as ei and ez and pulse fz caused by pulses ei, eg is, therefore, similar to pulse h in duration. Transition a4, however, is delayed to point b4 and' this pulse will occur when counter B'has counted more than oscillations and, therefore, counter A is not restarted. The output of the mixer circuit will consist of a single pulse es which as it corresponds in polarity to the last pulse ei, has no effect upon the trigger circuit.

The maximum distortion is now represented by the time difference between dv and cs and, if H6 no further telegraph signals are received, pulses from counter B will occur at instants ds, de, din and pulses from counter A at instants cs, c1o,rcn, the time nterVaIS da, ce; de, C10; d1o, C11, being constant, and giving rise to a series of pairs of pulses es, ev; es, eg; em, en, which are equally spaced by an amount equal to the maximum distortion, and which in turn produce output pulses fa, fi, fs from the trigger circuit. TheV output of the trigger circuit is applied to a direct-current meter in such a manner that the direct-current 'component of the wave-form of Figure if is indicated as in Figure lg. The reading of this meter is proportional to maximum total distortion and the meter is calibrated to read percentage distortion directly.

. A block schematic arrangement of one embodiment of the invention is shown in Figure 2 of the accompanying drawings. In this arrangement, the telegraph signals of which the distortion, if any, is to be indicated or measured, are applied to the input term'inals Ill, and are then'fed to a .differentiation network I I which produces at each signal transition a sharp pulse the polarity of which depends upon whether the transition is from mark to space or space to mark. These pulsesare then'modified by means of a selector .circuit [2 which functions to invert some of the.

pulses, so that all transitions produce a pulse of similar polarity.

The pulses thus produced are passed to a control amplifier, which comprises a trigger circuit [3 and an associated gate circuit I4. The trigger 'circuit isv a valve device having two conditions of stability, giving rise to two possible output currents, and which'responds to an inputk voltage to change from one to the other of the two conditions; The gate circuit is a transmission device, Vcontrolled by'the output current of the trigger circuit, so that together the trigger circuit and gate form the equivalent of an electronic switch.

The gate circuit is opened by the pulses from the selector circuit [2, and thereby allows oscillations to pass from a stable oscillation generator i5. This oscillator in this particular arrangement is chosen to have a frequency of T, when T is the telegraph frequency, so that each telegraph period corresponds to 100 oscillations, as previously described.. The oscillations are accordin'gly fed to two counting channels A and B sin parallel; counter A comprises counting stages l, ll, land IS, whichare respectively to scales of 5, 5, 2 and 2 and counter Bk similarly comprises stages 20, '2|, 22 and 23 also to scales of 5, 5, 2 and 2.

The oscillations from the oscillator are thus .applied to the counting channels when a signal is applied to the system and after every 50 oscilfllations 'counters l8 and 22 in the absence of any 'other-'signals applied thereto, emit a pulse, and counters |9 and '23 similarly emit a pulse after Vevery 100 oscillations. The pulses from counter 'l9 are applied to a mixer unit 24, and pulses from counter I'8 are applied to this mixer, but through a phase inverter 25; the output of mixer 24 accordingly' consists of pulses at half telegraph periods and of alternate polarity. Counting channel B is similarly arranged with a mixer 26 and phase inverter 21, but in this channel the phase'inverter is fed from the output of counter 23 so thatthe output pulses of mixers 24 and 26 are of opposite polarity.

The pulses produced by mixer unit 24 are ap- 'plied'to Va trigger circuit 23 which with gate cir- 'cuit'29 forms a-further switch device which will be alternately open and closed during successive halfV telegraph period intervals; the arrangem'ent is such that the switch is o'pened during the first half'of. each telegraph period interval following the start of ythe signal applied to terminals l0, and isclosed during-the second half. Mixer unit 26 feeds a similar trigger circuit 30 and gate Vcircuit 31 ;the switch soformed is arranged to be closed during the first half of each telegraph period interval and open during the latter half.

3 Gate circuit 29 iswarrangecl to control the apply- 7 fri-gi of-'the pul'se-output'ifrom selector l2 toaseries of buifer amplifiers132, 3'3, 34, 35, feeding respect-iv'ely' the' counters HL i'l,Y I'8i and [19; Simila-rl-y, gate' Circuit 312 controls' the feeding o'f't'he output from selector I2 to -a series of amplifiers 3%; 31,

3%;39' to? the countersZ-O, 21-,- 22-,. 234 respectively.

T-hei output of' the counter' iQ which, once the systern has: startedI operation response. toI an applied-signal and in' the ahsence of a: distorted si=gnalf,-- will consistof a series ofv pulses at tele*- gr-aiplr periods; is applied to' a phase invertr 40 which in turn feeds a mixer unit lill. To this mixer unit are fed. also' pulses from counter 23 which in the same circumstances consist of p'uls'e's similarly timed hut Which loy virtue' of the phase inve'rter lit-are of opposite polarity. The niix'er unit :ll feeds a trigger c'irc'uit 4-2, yof Which the mean output current is indicated rby a meter 43; so long as the input pulses to'the mixer Mv are-simultaneous-there is noeffect upon thetrigger Circuit and the meter 43 indicates' a steady or Zero current.

Reie'rring now tof Figure 1-, and assumin'g that thesignal such as that of the waveform is' applied' to* the system, the operation of the' ci-rcuit Will start at the transition bi, and counters A and B will start counting.- At transition b2, the selectorV Circuit iZ -produces an output pulse; 'but this, coi-nciding with pulses cs and dz -emitted Joy thecounters",- has no effect upon the output of the counters.-

A :further transition occurs at bs and occurs in the first half telegralph period interval following the output p-ulse of the counter A Which occurs atl cr. At this instant, gate circuit 29 is opened and gate c'irc'uit Si closed; there is raccordingly no effect, due to the transition ba, upon channel B;- 'outthe p'uls'ei is -fed to all the counters H5, I'I, fil-'and f which are; thereuporr, released and start to c'ount again as from the instant cs. The pulses s'ub'sequently applied to the mixer unit will, therefore, no longer coincide, but will occur lat spaced intervais such as ds, cs, giving rise' to output pulses ei, e2. These output pulses Will operatethe trigger Circuit to produce lan output cur- 'rentfi of Which the .duration is a measure of :the

distortion.

If |a trans-itionpoint, such as dv, precedes 'the 'relevant timing pulse, the channel B will be operated, while channel A is unaffected, and in similar fashion thev output current' of the trigger -circuit will be increased, as' at fa, in the manner previously described.

Figures 3a, 317 and 30 are circuit diagrams which are intended to be placed side lb-y side rto.

form one composite drawing, which is Ia circuit diagram of the major portion of the system shown in Figure 2, but omitting channel B. Channel B clesely resembles channel A, but for certain minor -alterations aprparent from Figure 2 and referred to hereinafter, and it is considered that the arrangement of channel B to complete the diagram of Figure 3 will be readily apparent. In Figure4 3, the parts corresponding to the blocks of Figure 2 hear like reference numerals, the chain dotted lines in Figure 3 dividing that fi'gure into its component blocks.

In Figure 3,- ithe input signals rare applied to terminals Ilt and are caused to operate a relay ett of which the Contacts 45 apply telegraph batterfy voltages 46, 41 across resistance RI. 'Ilhe voltage across resistance RI, which may be of the |form shown in Figure la or lb, is applied to the dierentiator circuit I I consisting of series condenser CI and shunt resistance. R.2. I The peaked voltagesthus produced are'applied to a valve VI of the selector -circuit I2 of which the load impedance is formed hy two resistances R3, RA included: in the anode |and cathode circuits respectively, so that the valve operates as a phase splitter, the voltages a-ppearing across these two resistances being of opposite phase.

I The anode of valve VI is c-oupled through .conden'sei'- G2' to' the grid ofv'alve V2 having a grid resistance H5 |and of which the cathode is retui ged! lto a! point ony a potential divider RB, RI

such that-the valve isbiased to or slightly beyond "the pointof anode current cut-off; in this way, negative-going pulses are suppressed in the grid ci-'r'c'uit of the valve, the output appearing on the anode load RQ being, therefore, only negativegoing pul'ses'.

In similar manner, the cathode of valve VI is coupled to a valve V3 .also b'iased to cut-off, and having the same ano'de load RB as valve V2. Negative-going pulse's in the grid circuit of Valve V3 'are similarly suppressed and the valve produces negative-going pulses across the anode load Ril. As valves V2 and V3 are fed in opposite phase,- howeve'r, the valves produce a negativegoing pulse irre'spective of the polarity of the pulse applied to .the valve V i.

TheI pulsesthus produced are applied 'G0 the input of trigger Circuit f comprising three valves V4, V5, V5. The operation of this triggercircuit i's explained in detail in our earlier application, to which attention is directed for a more complete description of its operation. Briefly stated, the valve V4 is hia-sed by connecting the cathode of the valve to a point on a potential divider RQ, R l 0, the valve having a grid resistance RI l. The voltage appear-ing :across the anode load Rl2 is directly coupled over resistance Ri'3 to the grid of the valve V5 and the voltage appearing across the anode load of this valve is applied over resistan'ce RM .to the grid of valve Vi. Valve V6 includes in its cathode l-circuit two resistances Rii, Rlfl, of whichy Rlt isyalso included inthe cathode circ'uit of Valve V5.

It can be shown that the valves V5 and V6 can adopteither Yof twov stable conditions; [one in which valve V5 passes a large anode current and valve V6 a low anode current and the other in which these conditions are reversed. The valves can he changed from one to the other of the two conditions by the application of an appropriate voltage to valve V4; further applications of a voltage Ofthe same polarity will not change [the condition of equilibrium.

The voltage -anpearing across resistance RIS, which will be high or low depending upon the condition of the trigger circuit, is used as a control voltage for a valve Vi formng the gate circuit M. This valve is biased hy returning its cathode to a point on a potential divider RH, R! 8 and Iincluding in the grid circuit resistance RIB and part of resistance RI 6. This valve is, therefore, arranged to loe biased with-in the normal range of lhias in one condition of the trigger circuit,A and to he basedbeyond cut-off in the other condition of the cirouift.

The gate circuit Iblthus formed is used to control the supply of oscillations from oscillator 55 to the two counting channels A and B, of which only the former is shown in Figure 3, Channel Acomprises the four stages f, ll, i and f), which are similar in general arrangement and in operation, and are of a type known per se. The first stage [6 comprises a pentode valve V3, the

screen grid and control grid circuits of which are V9V very' tightly coupled by a transformer TI so as to form an oscillator circuit in which the amplitude of oscillation would tend to be very large. The control grid circuit of the valve, Which includes the one winding of the transformer TI, is completed to earth through two rectifiers MI, M2 for example of the dry contact type, which are connected in the reverse' direction so that i the resistance of the grid circuit to earth .(in that direction) is represented -by the reverse resistance of the rectifiers in series and is, therefore, extremely high. If thermionic diodes are employed, the resistance can be considered infinite. v

The two rectifiers are shunted by a condenser C3 and the input through condenser C4is applied to the junction of therectifiers; the cathode of the valve V8 is given a positive bias with respect to earth by connecting it to a point on a potential divider formed by resistances B20, R2B'. A large by-pass condenser CE is included for the cathode circuit portion of the potential divider, and a suitable load resistance E26 is included in the anode circuit of the valve. g i

The anode potential of the valve V'I varies in accordance with the oscillations from the oscillator I and when the anode is at a low potential the condenser G4 is able to discharge 'through the rectifier M2 and the anode-cathode path of the valve. As the valve V'I is normally passing current, condenser C4 is normally discharged.

When oscillations are applied to valve V1, there is a resulting decrease in the anode current of the valve and the potential of the anode rises. A charging current accordingly flows into condensers C3'and G4, and the condenser C3 acquires a charge; the final potential between the plates of the condenser will depend upon the relative values of the two condensers and the maximum potential reached by the anode of the valve V'I. It is necessary for stable operation thatthe condenser 03 should attain a sensibly steady value while the anode remains near the peak positive potential.

When the oscillation again causes the anode current to increase, the condenser C4 will again discharge through the valve and the rectifier M2, but the condenser C3 will be unable to discharge either through the rectifiers MI, M2, since these present a high resistance in the-discharge direction or through the grid-cathode circuit of valve V8, since the resistance RZIJ is set to produce a large positive bias on the cathode of the valve. After one oscillation, therefore, the circuit is as it was in the initial condition, save that the condenser C3 has acquired a certain potential. The valve remains quiescent by reason of the prohibitive bias between grid and cathode.

When the second oscillation is received, the cycle of operation is repeated, but in this case the condenser G3 acquires a further increase of potential, though not quite as great as the first since the voltage available to charge the two condensers is diminished by the potential across condenser C3. I

i In similar fashion, each succeeding cycle of oscillation increases the potential across the condenser until a point is reached where the gridcathode potential of the valve, which is equal to the difference in potential of condensers 03.L

and C5 is such as to permit Lanode current to .flow in the valve.

As soon as this condition is reached, the valve oscillates and there is an immediate increase of anode current, accompanied by a large positive l' excursion of the grid. fl Heavy grid current is drawn and. the condenser CS acquires a large negative charge so that after the single pulsev of anode current, the initial conditions of the circuitare restored. It will be seen, therefore, that after a number of applied oscillations, a single pulse is'ernitted from the valve, and the number vof oscillations which are required to produce the single pulse can'L be controlled by adjustmentofthe bias on the cathode of the valve by means of the resistance B20.

The succeeding stages I'I, I8 and I9 comprise valve VII,4 VIII and VII which function in the same manner as 4the stage IG, except that stages I8 and lsfare to a scale of two, while stages IG and I'I are adjusted to operate to scale of five. Consequently, theoutput of valve VII) of counter IB is a pulse at each half telegraph period, and of valve VII of vcounter I9 a pulse at each complete telegraph period.

The 'output of valve VIII is fed to the inverter' stage 25, which comprises a valve VI2 arranged to operate as a conventional voltage amplifier. The output of Lthis valve is'fed to the first of two parallelL valves VII-I, VM, which constitute the mixer-stage 24. 'Valve VI3l is biased by cathode bias resistor kRZI and is provided with an anode load B22 which is common to the two valves. Valve VIII is biased 'also by Va cathode bias resistor B23, but receivesV its input from the valve VII of the finalcounter IS. i

The'voltage appearing across the-'anode load B22 will,,,therefore, consist of a series of pulses at half telegraph period intervals, due to valve VIS, and a series of pulses at complete telegraph period intervals, but of Aopposite polarity, due to valve V I4. By making the vlatter pulses larger in amplitude than theV former, the resultant output can be made to be a series of alternately positive and negative pulses at half telegraph period intervals. f

-This signal'is applied to the trigger circuit 28,

consisting of valves VI5, VIS and VI'I, which are arranged in the same way as valves V4, V5 and V6 of the trigger circuit I3, and operate in the same manner. In trigger circuit 28-, however, the anode current offlthe output valve VI'I Will have a waveform consisting of square waves of half telegraph period duration. This output current is applied to the valve VIII of the gate circuit 29; this valve is similar in operation to valve -V'I of gate circuitV VIII, except that gatercircuit 29 will be alternately open and closed during the first-half and latter half respectively of each telegraph period interval following the first transition of the applied signal. Gate circuit 29 controls the application of the pulse output of the selector circuit I2 to the input of Vthe buffer ampliflers 32, 33, 34, 35, which comprise respectively valves VIII, V20, V2I and V22. These valves, like valve VI2, are arranged as conventional voltage amplifiers, and the output voltages therefrom are fed respectively to an input circuit of the counter stage valves V8, V9, VII'I and VI I. As shown, the amplifier output is applied to the screening grids of the valves.

Asdescribed above, the gate circuit 29 is open during the first half telegraph period following the first transition of the applied signal, and during this and the alternate half periods, the gate circuit will permit a pulse generated by the pulse selectorA I2 to be applied to the counting valves V8, V9, VIII,-VI I. While the applied signal continues to be undistorted successive transitions will coincide with a pulse by the counting circuits and the transitionpulse applied to the counters Will have no effect upon the-countersA since they will, in any case, emit a pulse at that moment. When the transitionV o'ccurs at ba, however, ithe'counters will have all emitted a-pulse at the instant 04, which precedes it slightly andfwill have started to recount from that instant. Since the interval 04, cs is less than one half of a telegraph period, the gate circuit 29 is open and so there is fedto all the counters a pulse which discharges the charge, if any, upon the condensers^C3. vThe counters are accordingly restarted from the instant cs. v

From this instant, the'counter VH Will emit pulses at telegraph period intervals as before, and these pulses are fed through a phase inverter stage 40, comprising a valve V23 arranged as a conventional voltage amplifier, to a mixer stage 4| comprising the valves V24 and V25. This mixer stage is similar to the mixerstage 24 and lthe pulses from counter valve Vl l will accordingly appear in the common output of the valves V24 and V25. i

At this point, it is convenient to refer to the arrangement of the B channel, not shown in Figure 3. As appears from Figure 2, the general arrangement of this channel is similar to the A channel except that (a) the channel should be responsive to pulses from' selector circuit l2, which occur only in the second half of each telegraph period and (b) the output pulses from the mixer stage l corresponding to pulses from the final counter stage 23 should be of polarity opposite to those produced in the mixer stage by the counter stage IQ.

Channel B will, therefore, include stages substantially identical with those of channel A'except that the phase inverter valve VIZ and mixer stage valves VIS, Vl4 willbe fed differently so that resulting output from the mixer valves is in opposite polarity to that from the. valves. of the A channel. In this way, gate circuit 3] is made to close during the first half of eachV telegraph period and open during the latter half.

In channel B no phase inverter such as 40 is incorporated, so that the output pulses from the counters IQ and 23- will be applied tothe mixercircuit 41 with opposite polarity. x

Considering now the operation of the B channel in response to the signal transition which occurs at the point 05, it will: be seen that, sincethe transition occurs at a point whichis during the first half of a telegraph period, the gate circuit. 3l will be closed, and the countersi20; 21, 22 and. 23 will, therefore, be unafiected, and will produce an output pulse at instant ds. This pulse is applied to the input of the valve V251of the mixer stage, and will appear-.in the common output circuit of the valves V24 and V25. As previously described, the outputsrofA the channels A and B are now displaced in time in the manner indicated in Figures lc and ld and the output of the mixer stage will, therefore, be as shown in Figure le.

This last signal is applied to a trigger circuit 42, comprising valves V25, V21- and V28 which are arranged in the same Way as the valve of thev other trigger circuits l3 and- 28-." Theoutput of the valve V28 will, therefore, be ofthe form indicated in Figure 1 f, and by means-of a meter stage 43 comprising a meter 44 shunted1 by a large condenser C6, the average value of 'the anode current of the valve V28 can 'be indicated., The standing or'lesser current of the valve can be? backed-oif by connecting the meter V44V between lll a pulse is applied to differentiator Cl, RE, Which. 0

1:2 the anode 'of the valve V28 and a pointl on a potential divider E24.

The meter stage 43 will thus give an indication of the average value of the pulses developed by the trigger circuit 42; these pulses are of constant amplitude and With sharp leading and trailing edges, so that the average value is proportional to the duration of the pulses and so to the distortion of the telegraph signal by which they are produced.

It has already been pointed out that channel A- is in operation only during the first half of each telegraph period following the initial signal transition; it will, therefore, respond only to distortion Where a transition is delayed, and occurs after the instant at which the corresponding transition of an undistorted signal would occur. Channel B on the other hand operates only during the latter half of each telegraph period and will, therefore, respond only to that distortion in which a transition precedes the correct transition point, and the effect of the mixer circuit lil is to produce pulses spaced by an amount equal to the sum of the two distortions and is thus a measure of total distortion.

Once distortion has occurred and the counters of channel A have been restarted, the counters Will always restart counting at telegraph period intervals. until such time as. a transition occurs which is slightly more than a complete number of telegraph units after that transition Which last restarted the counters, i. e., until a signal cccurs which is more distorted (in the same sense) than any previous signal. Channel B operates in the same way for all signals which are distorted in the other sense, and of Which the. transitions precede the correct transition point.

Once distortion has occurred, the meter 43 will accordingly continue to indicate distortion until a greater distortion occurs. To cancel the indication means are provided to restart the counters of Channels A and B simultaneously. This is elfected by a key switch having ten contacts of Which four, Kl, K2, K3, Kl, are included in shunt to the condensers G3 of the counting valves V8, V9, Vi ll, VI I; so long as these contacts are closed, the valves will be unable to operate. Four further contacts are similarly arranged in the B channel counting stages 20, 2l, 22, 23. A contact KS is also arranged to shunt the resistances Rl5 and RHS in the cathode circuits of the valves V5 and VG of the trigger. When this contact is closed, the bias on valve VT of the gate circuit [4 is prohibitive and no oscillations are fed from the oscillator l5, and both channels A and B are completely quiescent, a similar contact being included inl the corresponding trigger circuit 3D. When the contacts of the key areopened, the tcircuit is rendered ready for operation, but the gate circuit Ill remains. closed until the first transition point of an applied signal occurs.

The operation of the circuit. of, Figure 3 Will be as' described with reference to Figure 2, but to recapitulate, the operation of the circuit in the presence of a signal as shown in Figure lb is as follows:

At, transition point bl relay 44 operates and is differentiated and applied to valve Vi; valves VI, V2 and V3 then operate as described above to produce a negative going pulse at the anode of V3. This is applied to trigger valves V4, V5

and V6, and the anode current of V6 changes from a high to a low value, thus opening gate ansaaszrvalve V1 and permitting oscillationslfronr oscillator to be fed-to the scale of, five counters =V8 and VS'and scale of two counters Vill and VI l. Counters 20, 2I, 22 and 23 are similarly started.

After 50 oscillations a pulse appears at the anode of valve V50, and corresponding pulse in counter 22. The pulse from valve VIII is applied.

to phase inverter V|2 and thence to mixervalve Vl3; the pulse from counter 22 is applied to mixer 2G. The pulse at the anode of mixer valve coincident with the generation of further pulses by counters IB (valve Vlfl) and 22, and by counters |9 (valve VI I) and 23, VPulses are produced in the common anode load of valves Vl3 and VI4, and a corresponding pulse in mixer 26. f'I'rigger circuits 28 change over, and gate circuits 23 (valve VIS) and 3| are respectively opened and closed. There is no output from mixer 4l.

At a point one half unit of time later gate circuits 29 and 3! are again reversed by the pulses generated at the counters 18 and 22 and at point cs a further half unit later the gate circuits again change over. At this point also pulses appear at the anode of valve VH of counter IQ and of the corresponding valve of the counter 23. These pulses being coincident there is again no output from mixer M. During the next whole unit up to cl this cycle is repeated.

At point ba however a further pulse appears at the anode of valve V6; at this instant gate circuit 29 is open and gate circuit 3| is closed, so that counters 56 to |9 are all discharged, but counters to 23 are unaifected as a result of this, at an instant one half unit after point cr gate 3l opens as before but gate 29 opens one half unit after ba (or cs). One whole unit after 04, counters 22 and 23 again emit pulses and that from counter 23 is applied to valve V25. A pulse is accordingly applied to trigger valve V25 causing valve V28 to increase its anode current from a low to a high value.

At point ce, one whole interval after ba, or cs a pulse appears at the anode of counter valves Vl, VI l and Vi l causes a pulse to be applied to mixer valve V24 through inverter valve 46, thereby changing valve V28 back to its low current conditions.

This sequence of operation continues until point bi is reached; at this moment gate circuit 29 will be closed, being in an odd half interval after transition ba but gate circuit 3! is open. Thereby counters 22 to 23 are discharged, and recommence counting. The output from counters 22, 23, which would have occurred shortly after, is suppressed, but at instant cs, one whole interval after cq pulses are again produced by valves VID and VI I. A pulse is then produced by mixer 4 I, but this has no effect on valve V28, since the pulse is of the same polarity as that last applied.

At point da, one unit interval after dv, a pulse is produced in mixer 4! to change the condition of valve V28, followed shortly after by a pulse ce due to valve VI l. Valve V28 will thus pass anode current for a short time, which is the desired indication of error.

It will be understood that While a specific form of the invention has been described, this has been given by way of example only, and that other 1.4 arrangements'may be adopted to carry the inveni tion into effect. 1

:We claim:

.1. A device for indicating or measuring the f; ditortinof a telegraph or nke signal consisting of elements each of which are, or should be, an integral multiple of'a telegraph unit, comprising comparisonmeans selectively responsive to said signal during a period immediately. following'a lwhole numberof units after a reference point of said signal, a second comparison means selectively responsive* to said signal during a period immediately preceding a whole number of units after a reference point of said signal, said comparison -means each Operating to produce a currentor voltage Which is a measure of the departure of said signal from the correct form, and indicating:

means responsive to said currents or voltages.

2. A device for indicating or-measuring the distortion of a telegraph or like signal consisting of elements each of which are, or should be, an integral multipleof a telegraph unit, comprising'a timing means rendered operative by the signal of which the distortion is to be indicated or measured and thereby to produce a series of timing voltages, two comparison means each responsive to said timing voltages and said signal, said comparison means being respectively cperative only during a period immediately preceding and immediately following a timing voltage, said comparison means each including means responsive to said timing voltages and to a change in said signal to produce an output current which is a measure of the difference in time occurrence of said change and a timing voltage, and means for combining additively the outputs of the comparison means to indicate the distortion of said signal.

3. A device for indicating or measuring the distortion of a telegraph or like signal consisting of elements each of which are, or should be, an integral multiple of a telegraph unit, comprising timing means initiated by a change of said signal to produce thereafter a series of timing voltages at telegraph unit intervals, a second timing means also initiated by a change of said signal and producing thereafter timing voltages at telegraph unit intervals, said timing means being arranged to be restarted respectively by signal changes which occur in a period immediately preceding or immediately following a whole number of telegraph unit intervals after said initiation, and means responsive to timing voltages from both said timing means for indicating the total distortion of said signal.

4. A device according to claim 3 and comprising an oscillation generator of a frequency which is a high multiple of the telegraph frequency, and said timing means each including means for deriving from the oscillations of said generator a timing voltage at telegraph period intervals by counting down said oscillations.

5. A device according to claim 4, Wherein the means for counting down said oscillations comprise a condenser the charge upon which is successively varied in accordance with successive oscillations, means for developing a timing voltage impulse and for restoring to an initial condition the charge on said condenser after a predetermined number of oscillations, and means responsive to a voltage derived from a signal change to restore said charge to its initial condition, whereby said counting means is caused to restart counting.

6. A device for indicating or measuring the distortion of a telegraph or like signal consisting of f' f is elements each of' which is., or shouid he, an inte gral multiple of a telegraph unit, Said'eViCe comL prising means for producing from the signal 'of which the distortion is to be measured a succession of control pulses corre'spondlin'g yeach toa transition or change of said signal, an oscillato'r whose ireque'ncy is a'hi'gh multiple of the telegraph frequency, means operated by said pulses to permit oscillations from said oscill'ator to pass to two counting devices, each counting device-im cluding means to produce from said oscllations output pulses at telegraph period intervals, two gate means controlled respectively by said counting means and arranged to be opened and closed during respectively the first and the second half of the telegraph period intervals as measured by the associated counting means, 'said gate means being arranged to control the application of said control pulses to said counting means and hen'ce the restarting thereof by said pulses.

7. A device according to claim 6, in combination with a trigger Circuit upon which said output polses from the tv'v'o said counting device-s afeaifi' plied ih 'opposite phase, said 'tf-lgger Circuit bei-hg of the type having 'two conditions of statility and being adapted to be 'changed -froi o'ne to the 'th'e'r' by voltages applied thereto, whereby said trigg'er c-ircuit develops 'an output voltage 'waveorxn "of constant amplitude and duration proportional to th'e idistortion 'of th'e original signal.

8. A-evi'ce according to claim 7, and comprlsing'meansfor indicati-ng the average value of said output pulses.

-EDWARD 'GI-IARLES PYATP. KENNETH CEDRIC ARTI-IUR KING.

REFERENCES GITED 'following reference: are of record in the file of this patent:

UN'iTED STATES PA'TENTS Number Name Date I 2,11022719 p instead .nr-...nerna Dec.- 21,193? 2137M85 v:Rea et ai. ,.--ll Main 63 1945 

